With recent advancement of digital technologies for electronic equipment, there have been increasing demands for larger-capacity and nonvolatile memory devices to store data of music, images, information, and so on. In one approach for responding to such demands, an attention has been focused on a nonvolatile memory device (hereinafter referred to as ReRAM (resistive random access memory)) including as a memory element a resistance variable layer which changes its resistance values in response to electric pulses applied thereto and retains the changed resistance values. This is because the nonvolatile memory device has a relatively simple structure as a memory element, makes it easy to provide a higher density, is highly compatible with conventional semiconductor process steps, etc. In this ReRAM, there is a need for materials which allow their resistance values to change with stability and with high reproducibility as designed in a miniaturized configuration of the memory element including the resistance variable layer, and establishment of manufacturing process steps thereof, and study and development for them have been made vigorously.
As a structure for enabling higher-dense integration, there is known a configuration, in which a memory cell including a memory element comprising a resistance variable layer and a two-terminal element having a non-linear current-voltage characteristic is provided in a region where a word line crosses a bit line (first Prior Art Example, e.g., see Patent Literature 1). It is recited that, in such a configuration, a switching characteristic (characteristic in which the non-linear element is placed in an electrically-conductive state or in a non-electrically-conductive state depending on whether or not an applied voltage exceeds a threshold) of the non-linear element can improve selectivity of a memory cell, and therefore, it is possible to implement a ReRAM having a higher density and being accessible at high-speeds.
There is also known a ReRAM implementing a more miniaturized structure. For example, there is known a memory device using a pore structure utilizing nano-holes (second Prior Art Example, e.g., see Patent Literature 2). In this memory device, a memory element including a lower electrode, a memory layer and an upper electrode, or the like, which are filled into a nano-hole structure formed by anode oxidization, is fabricated as the pore structure. It is said that this makes it possible to manufacture a ferroelectric element or a resistance variable element with a large effective area, and hence a semiconductor memory with a higher surface density and a larger capacity.